Effect of the RRA treatment on the aging characteristics and stress corrosion susceptibility of the commercial 7050 Al alloy were studied by measuring hardness changes and crack growth rates. The microstructural changes during RRA treatment have been investigated by transmission electron microscopy.
The metastable Gpzone solvus temperature for 7075 Al and 7050 Al alloys are determined to be respectively 160℃ and 180℃. The growth or nucleation rate of GPzone in 7050 alloy is determined to be greater than that in 7075 Al alloy. This is believed to be due to the high concentration of Cu in 7050 Al alloy.
The maximum retrogression time, within which the alloy recovers its original T6 strength after RRA treatment is significantly longer in the case of 7050 Al alloy than that in 7075 Al alloy. It is concluded that this si partly because of the effect of Cu on the nucleation and growth of GPzone and partly because of the influence of Cr, Si and Fe through their effect on the quench sensitivity of the alloy. This was born out by isothermal aging experiments at high temperatures.
The RRA treatments of the T6 tempered 7050 Al sample improve its stress corrosion resistance without sacrificing the original T6 strength. The crack velocity is reduced to a level comparable to that of the T73 case when the 7050 Al samples are retrogressed to maximum retrogression time within which the alloy recovers its maximum strength after reaging treatment. It is believed that the high concentration of particles, especially n' is responsible for the recovery of its maximum strength. The improvement of stress corrosion resistance is ascribable to the coarsening of the grain boundary precipitates which occures during RRA treatment.
The size of grain boundary precipitates as well as matrix precipitates are significantly bigger in 7050 Al case as compared to the 7075 Al case in all heat treatment conditions.
